The CUPS system is a printer spooler (i.e., it assumes that the printer is shared; it collects the stuff to be printed and sends it to the printer only when it is finished, in order not to delay other people's printouts when you send a page header, do 1/2 hour computing and then print the rest).

The easiest way around this would be to send the output directly to the printer device (that probably means first convincing CUPS that it should leave it alone, and making sure that the printer does understand the text sent to it).